Learn about your body's weaknesses in your 30s—and how to beat them—and you'll live a longer, healthier life

Unless you're one of the lucky few to hit it big in the genetic lottery, staying fit ain't easy. And if it's this difficult now, imagine what it's going to be like in 10, 20, 30 years. But before you begin planning your future as a wobbly, overweight hunchback, understand that getting in the shape of your life for the rest of your life doesn't have to be daunting.

In fact, if you find your fitness workout groove now, you'll set yourself up for success in the future. Studies show that working out may lower a woman's risk of breast cancer by 37 percent, osteoporosis by 45 percent, and heart disease by 14 percent. So we tapped dozens of doctors and fitness experts to find out exactly what you should do to add years to your life.

The bottom line: Whether you're fresh out of college or funding Junior's tuition, the moment to launch your lifetime fitness plan is now. Then simply adjust your total-body workouts throughout the years to give your muscles, bones, and heart what they need to keep working well enough to keep up with you.

Fate Says: Your Stamina is Flagging
Over the next 10 years, lazy ladies will lose up to 6 percent of their aerobic capacity. Good thing you're not one of them. Put your lungs and heart to work now—and in the decades to come—and you'll be an oxygen queen forever.

Cheat Fate: Do Integral Intervals
Those sprints you dreaded in high school are your BFF these days. Try this interval workout: Warm up with 5 minutes of walking. Next, sprint for 30 seconds, trying to reach 90 percent of your best effort. Then walk or jog lightly for 90 seconds. Repeat this sprint cycle 10 times. To make it harder, reduce your 90-second rest period.

Fate Says: Zero Time, Zapped Motivation
When researchers at Toronto General Hospital asked women in their 30s why they didn't exercise, 40 percent said they didn't have the time. Another 40 percent said they didn't have the willpower. Sound familiar?

Cheat Fate: Step On It
If you strap on a pedometer and start tracking your daily steps—and taking a few more of them—you'll begin to see results. University of Tennessee researchers found that women who took at least 10,000 steps per day had 9 percent less body fat by the time they were 50 than those who took between 6,000 and 9,999 steps. If you go for regular walks, try to add more steps in the same amount of time; you'll automatically improve your fitness level without increasing your exercise time.

If you're coming up short of 10,000 steps, start making small changes: Next time you go to the grocery store, carry only one bag inside the house at a time. (And try not to unpack the Baked Lays between the car and the kitchen.)

Fate Says: Your Back Is A Royal Pain
About 70 percent of women will report lower back pain at some point in their lives, and it often starts at age 30. During pregnancy, as many as 80 percent of women experience back pain.

The key to preventing it is not just muscular strength but endurance, too. "Your back muscles don't need to be overly strong to support your spine; they need to be able to support it for long periods of time," says Lori Incledon, author of Strength Training for Women. In other words, it's about how much force those muscles can exert as well as how long they can go before pooping out.

That’s doubly important when you’re expecting because your abdominal muscles stretch to make room for your growing uterus and can’t support you the way they usually do, which forces your lower back to bear the brunt. Not to mention your levels of the hormone relaxin are on the rise, loosening not only your pelvic ligaments (get that baby outta there!) but also the joints of your spine.

Cheat Fate: Shore Up Your Core
Do the side plank three times per week. Lie on your left side with knees straight and upper body propped on your left elbow and forearm. Place your right hand on your right hip and slowly raise your hips until your body forms a straight line from your shoulders to your knees. Hold this position for 5 to 10 seconds, breathing deeply. That's one rep. Repeat four times, then switch to your right side. Do one to two sets for each side.

If that's too hard, bend your knees 90 degrees so that they rest on the floor. If you're pregnant, do the exercise with your back against a wall for support.